1. Field of the Invention
The invention relates to a glass-ceramic composite encapsulation material, and in particular, to a glass-ceramic composite encapsulation material characterized in that it is prepared by incorporating at least one glass filler selected from the group of KAlSiO4, KAlSi2O6, and MgO in a Ba—B—Si—Al glass matrix, and is applicable to solid oxide fuel cell technology.
2. Description of the Prior Art
Among various fuel cell technologies, solid oxide fuel cells (SOFC) possess advantages such as high conversion efficiency, a wide selection of fuel sources, and lower material cost. However, its working temperature is very high (about 700° C.-1000° C.), and as a result, in the manufacture of a plate SOFC, a high temperature encapsulation technique is required to fill up interstices between plates with encapsulation materials so as to insulate fuel gas from oxygen. Therefore gas tight layers and joints are necessary between materials. Since the entire SOFC elements is operated under high temperature, under normal starting and stopping conditions, materials may experience temperature cycles ranging from normal and high. As a result, matching the thermal expansion coefficients of the various elements is important. Consequently, some types of encapsulation materials play a role as a buffer to reduce damage to the elements caused by the stress generated from heat expansion and cold shrinkage.
FIG. 1 shows the comparison between the inventive formulation and same similar literature formulation. Glass composition used by K. Eichler was based on Pyrex 7059®. U.S. Pat. No. 6,430,966 disclosed a similar glass formulation but contained no B2O3, and it is apparent that ranges of its ingredients are: SiO2, 45-70 mol %; Al2O3, 5-15 mol %, and BaO, 20-50 mol %. Since the softening point of this type of glass is relatively high (780-860° C.), the process of encapsulation requires an increase of temperature up to 1000° C. As a result, materials that use Yttria-Stabilized Zirconia (YSZ) as an electrolyte are considered suitable materials for use in SOFC as these materials operate at a temperature higher than 800° C. In the future, the operating temperature of SOFC will be at about 750° C. or even less, and hence less expensive steels may be selected to be materials of the shell and bipolar plates to further reduce production costs. In contrast, higher temperature Pyrex 7059® glass is unsuitable because of its material cost in manufacture at higher temperatures.
When a glass material is used to seal a solid oxide fuel cell, the mechanism for how to add oxides to lower the softening point of the glass to match the thermal expansion coefficients of the other materials to be encapsulated has not been sufficiently tested. The difference in the expansion coefficient of encapsulation glass and materials to be encapsulated results cracks to form in the barrier. In addition, the use of alumina, boron oxide, barium oxide, silica, kaliophilite (KAlSiO4), leucite (KAlSi2O6) and magnesium oxide (MgO) in the material have not been adequately tested.
In view of this, the above-described conventional art has many disadvantages and requires a solution.
In light of various disadvantages derived from the above-described methods, the inventor had devoted to improve and innovate, and finally, after studying intensively for many years, successfully developed a glass-ceramic composite encapsulation material according to the invention herein.